While this is funny, the real answer is Soundscan going live in the late 1990s.
Before then, no one really knew what music was popular, and the labels tended to put a thumb on the scale and inflated the popularity of their pet rock and pop acts. Soundscan revealed the hip-hop/rap and country were much, much more popular among actual consumers than anyone thought.
That resulted in more dedicated C&W and hip hop radio stations, and the general decline of rock, which all became self-reinforcing.
Tl;Dr there's a lot more Panderin' fans out there than anyone thought.